Recently, a device is coming into wide spread in which image information is treated as digital and an image is compressed and coded by adopting a coding method which compresses using orthogonal transformation such as a discrete cosine transformation and movement compensation using the redundancy which is a characteristic of image information with the transfer and accumulation of information with high efficiency being an object at this time. In the coding method, for example, there is MPEG (Moving Picture Experts Group) and the like.
In particular, MPEG2 (ISO/IEC 13818-2) is defined as a general image coding method and is a standard which includes both interlace scanning images and sequential scanning images, standard resolution images, and high-precision images. For example, MPEG2 is currently widely used as an application with a wide scope for professional purposes and consumer purposes. Due to the use of the MPEG2 compression method, a coding amount (bit rate) of 4 to 8 Mbps is allocated if using, for example, the interlace scanning images with standard resolution which have
720×480
pixels. In addition, due to the use of the MPEG2 compression method, a coding amount (bit rate) of 18 to 22 Mbps is allocated if using, for example, the interlace scanning images with standard resolution which have
1920×1008
pixels. Due to this, the realization of high efficiency and excellent image quality is possible.
MPEG2 mainly has high image quality coding which is appropriate for broadcasting as a target, but is not supported in the coding amount (bit rate) which is lower than MPEG1, that is, the coding method with a higher rate of compression. Due to the diffusion of mobile phones, it is thought that the needs of a compression method such as this will increase in the future and the standardization of an MPEG4 coding method is being performed according to this. In relation to the image coding method, the specifications of ISO/IEC 14496-2 of December 1998 has been recognized as the international standard.
As the schedule for standardization, there is an international standard of H.264 and MPEG-4 Part 10 (Advanced Video Coding which is referred to below as H.264/AVC) in March 2003.
Furthermore, standardization of FRExt (Fidelity Range Extension), which includes coding tools which are necessary for administration with RGB, 4:2:2, and 4:4:4 and a 8×8 DCT and quantization matrix which are specified by MPEG-2 as an extension of H.264/AVC, was completed in February 2005. Due to this, FRExt became a coding method where it is possible to excellently realize film noise which is included in video using H.264/AVC and become a transport which is widely used in applications such as Blu-Ray Discs (registered trademark).
However, recently, needs with regard to further high compression rate coding are increasing such as the compression of images with approximately
4000×2000
pixels which is four times that of high vision images and the distribution of high vision images in an environment of limited transfer capacity such as the Internet. As a result, studies in relation to the improvement of coding efficiency are continually being performed in VCEG (=Video Coding Experts Group) under ITU-T as described previously.
As one improvement in coding efficiency, a method is proposed where there is a FIR filter in a movement compensation loop (for example, refer to NPL1). It is possible to suppress deterioration of a reference image to a minimum and improve the coding efficiency of image compression information which is the output by determining the FIR filter coefficient in a coding device so that an error between the FIR filter and the input image is minimized using a Wiener Filter.
Then, with the object of further improving the coding efficiency using H.264/AVC, currently, standardization of a coding method which is referred to as HEVC (High Efficiency Video Coding) is progressing due to JCTVC (Joint Collaboration Team—Video Coding) which is a collaborative standardization body of ITU-T and ISO/IEC. NPL2 has been published by HEVC as a draft.
A coding unit (CU) has been defined in HEVC as the same processing unit as that of the macroblock in AVC. The CU is designated in the image compression information in each sequence without the size being fixed to
16×16
pixels unlike the macroblock in AVC.
The CU is configured in a layered manner from the LCU (Largest Coding Unit) which is the largest to the SCU (Smallest Coding Unit) which is the smallest. That is, in general, it is possible to consider that the LCU is equivalent to the macroblock of AVC and the CU in the lower layers than the LCU (the CU which is smaller than the LCU) is equivalent to sub-macroblocks of AVC.
Here, a method is adopted in HEVC with an adaptive offset filter which is proposed in NPL3. The adaptive offset filter in HEVC is provided between a deblock filter and an adaptive loop filter.
As types of adaptive offsets, there are two types which are referred to as band offsets, six types which are referred to as edge offsets, and furthermore, it is possible that the offsets are not adapted. Then, it is possible to divide the image into a quad-tree and select whether to code using which of the types of adaptive offsets described above in each of the regions. It is possible to improve the coding efficiency using the method.